The field of data communications has been in turmoil during the recent years. New technologies are being introduced while old technologies are being dismantled. Particularly, the data rates in wireless mobile communication systems have been increasing in the recent years rapidly. Long-Term Evolution (LTE) standardized by the 3G Partnership Project (3GPP) represents a significant leap forward in wireless mobile communication systems. One of the main objectives of the LTE is the providing of downlink data rates of at least 100 Mbps and uplink date rates of at least 50 Mbps. The LTE operates in two modes, namely the Frequency Division Duplex (FDD) and the Time Division Duplex (TDD). In FDD the uplink and downlink transmissions use different frequency bands, which are separated by a frequency offset. Thus, the FDD operates in paired frequency bands. From a mobile node, that is, user equipment perspective there are two carrier frequencies one for the uplink transmission and another for the downlink reception. The downlink reception and uplink transmission occur simultaneously. The downlink reception uses the Orthogonal Frequency Division Multiple Access (OFDMA) while the uplink transmission uses the Single Carrier Frequency Division Multiple Access (SC-FDMA). The reason for the use of SC-FDMA in uplink transmission is the high Peak-to-Average Power Ratio (PAPR) in OFDMA signal transmission. An amplifier in an OFDMA transmitter must stay in amplifier linear area by using extra power back-off. This leads to increased battery consumption or shorter uplink range. The shorter uplink range may be a problem for mobile nodes that are far from a base station. In FDD the problem is the required availability of enough radio spectrum for the paired band. Therefore, TDD has been standardized as an alternative for FDD. TDD uses the same frequency band for transmission and reception so that the base station and the mobile node take turns in transmission. TDD emulates full-duplex transmission in a transmission which is essentially half-duplex in nature. This is possible because of the rapid change in the transmission direction.
The possibility for device-to-device communication between end-user terminals has been discussed in LTE standardization. The use of device-to-device communication provides certain advantages. Firstly, it allows the saving of uplink and downlink radio resources, for example, OFDMA or SC-FDMA resource blocks to be freed and thereby increases the overall capacity available for users that cannot harness device-to-device communication for their own purposes, for example, if they are downloading files from a distant server. Secondly, device-to-device communication may take place within frequency bands having a transmission power upper limit such as TV channels adjacent to an operational TV channel in TV White Spaces (TVWS). Thirdly, device-to-device communication may be used to increase the coverage area of a base station cell to areas where it would otherwise not reach, for example, to tunnels, caves and buildings not equipped with base stations. This is achieved by the use of chains of end-user devices that end to a device which is actually within the coverage area of a base station cell. Fourthly, the use of the use of device-to-device communication requires less transmission power compared to normal uplink transmission and that in turn reduces battery drainage. Fifthly, in some cases device-to-device communication may provide better radio channel quality, which in turn leads to improved Quality of Service (QoS) for users.
In order to be able to establish device-to-device communication an initiating device must request a radio resource for probing the target device. The probing involves transmitting a test signal to the target device. If the quality of the test signal is sufficient, the devices may start communicating using a radio resource for device-to-device communication. If the test transmission fails to reach the target device or the quality is too low, the devices must communicate conventionally via a base station and a core network. Obviously, such an event-triggered device-to-device communication set-up process involves a lot of signaling overhead to the cellular system and a long communication establishment delay as well, especially when test transmission fails. The establishing of a multi-hop device-to-device communication path may involve a significant delay.
It would be beneficial to be able to establish beforehand information on possible device-to-device communication routes to a mobile communication system, for example, in areas where device-to-device communication occurs frequently. The pre-established information would speed up the process of establishing device-to-device communication routes. The communication routes may involve a number of hops via different end-user devices or the routes may just be direct connections between two end-user devices. More benefit is achieved if the routes include multiple hops.
In the area of ad-hoc networks, which refer to networks including mobile devices as potential routers, certain routing methods have been developed. In pro-active routing methods such as the Highly Dynamic Destination-Sequenced Distance Vector Routing Protocol (DSDV) routing tables are maintained on each node acting as a router. The routing tables are constructed using route advertisements distributed by all nodes in the network. The routing tables include information on destinations, next hops, distance and sequence numbers to determine the age of route information. The disadvantages of pro-active routing methods include the amount of data and the number of route advertisements that must be exchanged by nodes for the maintenance of up-to-date routing table. The pro-active routing methods are also slow to react to ad-hoc network topology changes and inter-device link failures. Other examples of pro-active routing methods include Ad-hoc Wireless Distribution Service (AWDS), which is a layer 2 wireless mesh routing protocol, and Direction Forward Routing (DFR).
By contrast, there are source-initiated on-demand protocols, which may also be called reactive routing protocols. An example of such a protocol is the Dynamic Source Routing Protocol (DSR) for mobile ad-hoc networks for IPv4. A source-initiated on-demand protocol finds a route, when sending a packet to a new destination, by flooding the network with route request packets. The route request packets record the nodes traversed. The destination replies with a route reply packet containing the strict node-by-node route to the destination. The disadvantages of such algorithms are high latency time in the finding of routes and the fact that excessive flooding of route request packets can lead to network clogging. Another example of a source-initiated on-demand protocol is the RSRP: Robust Secure Routing Protocol (RSRP) for mobile ad-hoc networks.